This invention relates to a variable stator cascade for axial-flow turbines of gas turbine engines, especially for high-pressure turbines, where the turbine nozzle vanes are each pivotally arranged about an axis of rotation and are cooled when in operation.
Theoretical considerations have shown gas turbine engines with variable turbine stator cascades provide a number of desirable properties over gas turbine engines with rigid stator cascades. Analysis of the existing relationships has indicated a number of advantages. For example:
1. Improvement of the part-load consumption of heat exchanger gas turbines (fixed compressor turbine, variable power turbine). PA1 2. Improved life of components subject to low cycle fatigue (LCF) of material fatigue produced by low-frequency pulsating load and thermal shock, improved acceleration performance and part-load consumption for jet engines and turboshaft engines without heat exchangers (variable compressor turbine and perhaps variable power turbine). PA1 3. Changes thrust performance and specific fuel consumption of jet engines. Such engine, known as variable-cycle engines (VCE) in the Anglo-American literature, exhibit various combinations of variable high-pressure, intermediate-pressure and low-pressure turbines, depending on the degree of complexity of the engine.
Previously implemented or disclosed proposals for variable turbine stator cascades almost invariably involve uncooled stator cascades of low-pressure turbines, where the nozzle vanes are all pivotally arranged and where each nozzle vane is carried by one pin only in the outer ring of the stator cascade. In this arrangement, the inner ring of the stator cascade does not form a structural member of the stator cascade but must be supported by an additional, spider-type support. This support, serving as it does no function in the conversion of the energy in the working medium into useful energy, inherently causes flow losses, and the temperature of the working medium may be such that this component also requires cooling. A further disadvantage embarrassing this construction of the variable stator cascade is the fact that, regardless of the vane position at the time, the clearance between the vanes and the inner ring of the cascade varies as a result of different thermal expansion of the variable vanes and the outer ring of the cascade on the one hand and of the spider support and the inner ring of the stator cascade on the other, causing additional losses in the process. A further disadvantage of this construction of the stator cascade is the limited supply of cooling air flowing to the vane through the pivot pin, which disqualifies it for use as a turbine inlet stator cascade in engines exhibiting elevated turbine inlet temperatures.
Gas turbine engines having variable turbine nozzle vanes have been disclosed by, e.g., U.S. Pat. Nos. 3,919,890, 3,224,194, 3,558,237 and British Pat. No. 951,298. Within the problem area here discussed, however, there is in these teachings no showing or suggestion of the present invention.